I'd read that the only upside of a loft aerial is that it will last longer than one outside - if it's in anyway analogous to a mobile signal then mine is awful in the loft but fantastic if I stick my head out of the loft window.
We'll probably go for the loft option but I can't shake the idea that the signal is going to be much better without a roof surrounding the aerial.
The general logic was roof = best, roof in the way = worse, wall in the way = worse again, loads of walls = the worst.